Re: Erratic failure to specialize function

On Mon, Dec 19, 2011 at 12:26 AM, Antoine Latter wrote:

On Sun, Dec 18, 2011 at 2:57 PM, Alexey Khudyakov wrote:

...

Hello!

I've found a puzzling performance problem with code which uses vector library and relies heavily on GHC to perform inlining and specialization. In some cases compiler refuses to specialize function and just copies there generic version which is slow.

Here is smallest test case I've manages to make:

This is a guess, but based on what I've read the GHC inliner only fires when the function is fully saturated as declared - so if you declare a function with one argument to the left of the '=' symbol, the inliner only then inlines when it is applied to one value.

This means that the un-inlined function is passed to criterion in the first case, but not the second.

Yes. That's the case. I've checked the core and indeed in the first case function wasn't inlined.

Does adding a SPECIALIZE pragma help?

Yes. Since GHC have a optimized version it chooses it. So it looks like that functions which must be inlined (e.g. to eliminate dictionaries) should be written with no parameters

variance = \vec -> G.sum vec

This variant gets inlined. Otherwise SPECIALIZE pragma could help. Although it's annoying to write so many pragmas